In recent years, surface mount technology with increased density has been widely adopted. Such surface mount technology are classified into double-sided surface mount technology in which chip type parts are joined with use of solder paste, and hybrid mount technology which is a combination of surface mount technology of chip type parts using solder paste and through-hole mount technology of discrete parts. In either mount process, a printed wiring board is subjected to two or more soldering steps, and thus it is exposed to high temperature resulting in a severe thermal history.
As a result, oxide film formation is accelerated by heating the metal surface of copper, a copper alloy, or silver of a metal conductive part constituting the circuit part of a printed wiring board, and thus the surface of the conductive part cannot maintain good solderability.
In order to protect the metal conductive part of the printed wiring board from air oxidation, a treatment of forming a chemical layer on the surface of the conductive part is widely performed using a surface treating agent. It is necessary, however, that good solderability be maintained by preventing the chemical layer from degenerating (i.e., being degraded) to protect the metal conductive part even after the metal conductive part has a thermal history of multiple cycles.
Since an imidazole compound has an excellent film-forming property, as such surface treating agents, water-soluble prefluxes containing various imidazole compounds have been proposed. For example, Patent Documents 1 to 4 disclose 2-alkylimidazole compounds such as 2-undecylimidazole, 2-arylimidazole compounds such as 2-phenylimidazole and 2-phenyl-4-methylimidazole, 2-alkylbenzimidazole compounds such as 2-nonylbenzimidazole, and 2-aralkylbenzimidazole compounds such as 2-(4-chlorophenylmethyl)benzimidazole.
Incidentally, since an imidazole compound is generally hardly soluble in water, it is necessary to dissolve it in water using as a solubilizing agent an organic acid or an inorganic acid which forms its water-soluble salt.
In recent years, there has been widely used, as a solubilizing agent, formic acid or acetic acid which has both of an excellent performance to dissolve an imidazole compound in water and an excellent performance to bring out a film-forming property of the imidazole compound. However, since these acid compounds are highly volatile and have irritating odor, there arise problems that the imidazole compound may precipitate owing to decrease in acid concentration in a water-soluble preflux and the acid compounds may invite deterioration of working environment.
Tin-lead alloy eutectic solders have been widely used in the aforementioned soldering. In recent years, however, concerns have developed that the lead contained in the solder alloys adversely affects the human body, and thus the use of lead-free solders is desired.
Accordingly, various lead-free solders are being investigated. For example, lead-free solders have been proposed in which a metal such as silver, zinc, bismuth, indium, antimony, or copper is added to a base metal of tin and a part thereof is put into practical use.
The conventionally used tin-lead alloy eutectic solders are excellent in wettability on the surface of a metal such as copper, a copper alloy, or silver of a metal conductive part, and thus strongly adheres to the metal, resulting in high reliability.
In contrast, a lead-free solder is inferior to the conventionally used tin-lead solders in wettability on a metal surface, and thus exhibits poor solderability and low bonding strength due to voids and other bonding defects.
Therefore, when using a lead-free solder, it is necessary to select a solder alloy with superior solderability and a flux suitable for the lead-free solder. A water-soluble preflux for use in preventing oxidation on the surface of a metal such as copper, a copper alloy, or silver of the metal conductive part is also required to have functions for improving the wettability and solderability of the lead-free solder.
Moreover, most of lead-free solders have a high melting point, and a soldering temperature is about 20 to about 50° C. higher than that of the conventionally used tin-lead eutectic solders. Thus, the water-soluble preflux is desired to have the characteristic of being able to form a chemical layer with excellent heat resistance.    Patent Document 1: JP-B-46-17046    Patent Document 2: JP-A-4-206681    Patent Document 3: JP-A-5-25407    Patent Document 4: JP-A-5-186888